The Nebula Distortion is a clone of the Maxon DS-830 Distortion Master, a high-gain entry in Maxon’s line of pedals. It’s got a lot in common with the Ibanez MT-10 Mostortion, the main difference being an active two-band tonestack which uses simulated inductors (gyrators) in place of the MT-10’s passive 3-band tonestack which is more like what you’d find on a Fender or Marshall amplifier. The DS-830 also has a hard-clipping stage after the feedback-loop clipping.
Due to the size of the circuit, I wasn’t able to fit any switch mods like for clipping diodes. I did include a Softness trimmer like on most of my other PCBs that use hard clipping. By turning up this control, you reduce the effect of the hard clipping diodes and round it out a bit. You can read more about this control on AMZ (Jack Orman calls it “warp” rather than “soft”).
The DS-830 doesn’t exactly have a rich history behind it. It was released in 2001 as part of Maxon’s “Vintage Series”, a line of newly-designed analog pedals that fused vintage and modern circuit design, utilizing Maxon’s 30-year expertise in the field of analog effects to create some new classics. (The OD-820 Overdrive Pro was also released as part of this series.)
Despite being a relative late-comer, the circuit is a very worthy addition to Maxon’s product line and retains the spirit and character of their classic designs: a JRC4558D op-amp with feedback-diode clipping like the Tube Screamer, plus some interesting twist that makes it unique. In this case, it’s a two-band gyrator tonestack (not dissimilar from a Boss HM-2 Heavy Metal) that offers a great deal more flexibility and imparts a very different tonal character than the typical amp-like passive tonestack.
Input buffer → gain/clipping stage → hard clipping → 2-band EQ → volume control & output buffer
After we get past the standard unity-gain transistor buffer, there’s a clipping stage nearly identical to that of the Tube Screamer. It uses symmetrical diodes in a feedback clipping configuration. The main difference is the R-C filter coming off the inverting input of the op amp. The Tube Screamer has a corner frequency of 723 Hz, while the DS-830’s is 408 Hz, meaning that it has a lot more bass coming through at that point.
Immediately after this stage is a diode pair going to ground, which creates hard clipping. Since the clipping threshold is the same as the one in the previous stage, there won’t be much signal to clip off at this point—although with identical diodes, a feedback clipping configuration will retain slightly more of the signal than hard clipping, so it’s definitely doing something.
Next up is the 2-band EQ, which uses gyrators (simulated inductors) to boost or cut a specific frequency. These types of EQ controls are often seen in graphic equalizers because they can target a specific frequency and leave everything else alone. You could keep adding more of these with different tuned frequencies until you had a 6-band EQ, and you wouldn’t have any issues like you would with an amp-like tonestack.